1. Field of the Invention
The present invention relates to an apparatus and a method for removing interfering pulses from modulated or unmodulated video frequency signals. More particularly, it automatically and dynamically adjusts its threshold for detection and elimination of interfering noise pulses, e.g. from a neighboring radar transmitter, from a radiometric information signal.
2. Description of the Prior Art
One method for eliminating interfering signals caused by neighboring radar systems from a receiving radar antenna is through use of phase splitter, polarity and amplitude control and amplitude gates to cause the interfering pulses to cancel themselves out of the echo presentation or display device of the receiving system. This method focuses upon accomplishing an elimination of such interference in a qualitative manner and not in the carefully controlled quantitative manner needed for radiometer receiver interference.
Another method employs the heterodyne principal under conditions of strong interference by shifting the frequency of the system receiving the interference so that the signal to be measured is proportional to the weak signal of interest and not the strong interference signal. This method requires the operation of all circuits and elements to be linear regardless of signal amplitude and also requires active adjustment of the frequency of operation of the transmitting-receiving system subject to the interference.
Still another method which is directed to a combined frequency modulated radar and radiometer system uses a reference system including a dual frequency modulated oscillator which functions as both a radar transmitter and as a radar radiometer local oscillator. Such a reference system includes a switch which alternately connects a receiver to a reference termination and to an antenna. The radiometer used in the reference system is generally of the "Dicke" type modified to include a radar capability. Such a system provides for the broad concept of both a radar and a radiometer capability in combination. However, the ability of the reference system to eliminate radio frequency interference is a disadvantage in some applications.
Still another method relates to a frequency-scanning receiving systems' ability to reject clutter or interfering noise signals, e.g. doppler returns, in the process of locating the presence of the desired signal energy. The method utilizes a swept oscillator to detect undesired clutter signals and to establish a means for limiting the swept frequency range to eliminate the clutter signals. In addition to the need for frequency sweeping, the system is limited to CW type operation.
Another known method is directed toward muting AM noise occurring between incoming information carrying signal frequencies as the AM signal frequency is being tuned from one information carrying frequency to another. The method uses a phase lock loop which when locked on a carrier frequency causes the muting device to deactivate. This method is frequency sensitive and is not designated to handle pulses of interfering noise.